yesterday, one person gave answer in great details to one of my question and that made several things clear to me and satisfied.

I wanted to say thank you to him, so I thought to give 5 points bounty to him.

But no. I found that I can't give bounty until I have 75 reputation, nor can I give less than 50 point bounty.

I want to know the merit of both of these restrictions.

I was a have-not in scale of reputation, having 48 reputations of my own, so I just can't give 50 point bounty. Does it mean I don't have a right or a reason to show common courtesy to someone's useful help to me?

And before I reach 50 reputation, I don't have a right or reason to think of finer things like giving small bounties as a gesture of gratitude?

I am finding myself confined that I get denied these small things.

I think it should be allowed to give bounties in multiples of 5, starting from 5 bounties.

And as soon as one has 5 reputation, he should be able to give bounties, even if that means he ends up giving his entire 5 points to someone, of course he is doing it for a good reason.

Seems most or all of decision-makers of this site have a huge stock of bounties so they are not able to appreciate the feelings and compulsions of us "bounty-have-nots".

Giving an up vote is completely free to you, and is more of a bonus than a single bounty of 5 reputation.

As bounties can only be given to answers that makes your proposed 5 rep bounty worth less than a single upvote.

There is no reason to give small bounties when the reputation system does it naturally, thus a minimum bounty of 50 signals a "this is actually good" intention beyond what a normal upvote is doing.

Being able to give 5 rep would be a slap in the face. "I didn't want to upvote your answer, which costs me nothing, so here's 5 rep to ride you over instead."

Bounties are intended as a way to draw attention to questions, to promote further answers or improvements to existing answers.

An effect of placing a bounty on a question is to put it into the featured category for 7 days where it can draw further attention prior to the bounty being awarded.

Bounties are not normally intended to be just a public "thank you" to the author. They are intended purely for raising some serious attention to a question. While there is a bounty reason of "one of these answers is exemplary" it still results in a timed bounty.

There is no intention for bounties to be used as a casual way to pay reputation to users for answers. Being able to casually disperse reputation in this way could lead to abuse of the reputation system.

Bounties on the site mirror real life bounties in that people put up money for something to be done in the future.

Perhaps you are suggesting the option for a donation system, but again that leaves too much open for abuse where a nefarious user could gain control of a second account and siphon reputation to their own ends.

The current system is designed to provide the most public attention possible and keep all transactions metered, monitored and fair to everyone.